In the field of hydrostatic machines, e.g., excavators, variable displacement hydraulic pumps are typically driven by a prime mover, providing hydraulic power to a plurality of work implements as well as to the drive system. The variable displacement hydraulic pumps deliver power to the drive system via at least one variable displacement hydraulic motor.
In earthmoving machines which use hydrostatic transmissions, one of the problems that occur is that the machine will be operated in an overspeed condition. An overspeed condition occurs when the machine is allowed to accelerate, e.g., down a slope, to a speed at which the hydrostatic transmission actually tries to drive the drivetrain. That is the hydraulic motor will act like a pump and the hydraulic pump will act like a motor. This will put undesirable stresses on the drivetrain and the engine in particular. An overspeed condition can thus cause severe damage to the pumps, motor and engine.
One attempt at overcoming this problem uses engine speed. This scheme monitors engine speed and corrects for overspeed conditions. However, such systems are reacting to overspeed conditions and do not eliminate the risk of damage to the drivetrain.
Some systems attempt to correct overspeed conditions by controlling the displacement of the variable displacement hydraulic pump. These types of systems tend to cause instabilities or oscillations in the pump.
The present invention is adapted towards overcoming one or more the problems as set forth above.